HNS has been prepared by oxidation of TNT. However, the process has not been entirely satisfactory since the yields based on the starting material, TNT, have been uneconomically low.
Thus, HNS can be prepared according to the process disclosed in U.S. Pat. No. 3,505,413 by the action of sodium hypochlorite on TNT at 15.degree. C. in tetrahydrofuran/methanol solution. The yield of HNS obtained by this method is typically about 30-35% after acetone washing to remove the bulk of coprecipitated impurities, chiefly dipicryl ethane, also known as 2,2', 4,4', 6,6'-hexanitrobibenzyl (HNBB). In addition, the crude HNS produced in about 40-45% yield contains large amounts of an impurity, the so-called "red oil," believed to consist chiefly of trinitrobenzyl chloride, trinitrobenzaldehyde, trinitrobenzylalcohol, trinitrobenzoic acid, and trinitrobenzene, which causes difficulty in separating the HNS product and reuse of the tetrahydrofuran product.
This process is also described in Shipp et al, J. Org. Chem. 31, 857 (1966).
In an effort to improve yields, Salter et al, British Pat. Application No. 76/2501, Jan. 22, 1976, U.S. Pat. No. 4,085,152, treated TNT in tetrahydrofuran/methanol with aqueous sodium hypochlorite at a temperature of about 10.degree.-20.degree. C., then added an aqueous solution of an organic amine, preferably trimethylamine. The yields of HNS produced by this process are about 50%.
Kompolthy et al, Hungarian Pat. T/9639 No. VE-719 (CO6 f 9/04) developed a new procedure based on the air oxidation of TNT. They also observed that the preparation of HNS from TNT could be done in two steps as follows: ##EQU1##
Shipp and Kaplan, Heat Resistant Explosives XVI, NOLTR 64-63, (1964) showed that TNT could be converted to HNBB or HNS using sodium hypochlorite under varied conditions, but they did not demonstrate any procedure for converting HNBB to HNS. Shipp and Kaplan obtained a 79% yield of HNBB from TNT.
Kompolthy et al obtained an 82% yield of HNBB and reported yields of 76-91% of HNS from HNBB using dimethylformamide or dimethylsulfoxide as solvents in a reaction mixture containing methanol, potassium hydroxide, copper sulfate and pyridine. This Kompolthy et al work has been repeated by others but yields of only 25-40% of HNS have been obtained.
There is, therefore, a need for a process for the production of HNS in high reproducible yields.